1. Field of the Invention
The present invention relates to a method for producing a powdery and granular polymer having a sharp particle size distribution and a high bulk specific gravity.
2. Description of the Related Art
Hitherto, various investigations have been made on a method for improving the powder characteristics of a powdery and granular polymer obtained in recovering the polymer from a polymer latex. For example, U.S. Pat. No. 4,897,462 discloses a method of coagulating a graft copolymer latex containing many elastic backbone polymers at the particular concentration of a coagulating agent. Also, a method of adding a coagulating agent in two or more steps, and a method of adding a hard resin, inorganic compound, etc. for modification to the resulting coagulated particle are disclosed in Japanese Patent Application Kokai No. 59-91103 and U.S. Pat. No. 4,275,178.
In the method of coagulating at the particular concentration of a coagulating agent, there is a case where the coagulation does not occur at all depending upon the kind and amount of a surface active agent present in the latex, or a case where a powdery and granular polymer having improved powder characteristics is not obtained even if the coagulation is carried out so that the uncoagulated latex remains in only few amounts. Therefore, an acid or salt is added in large amounts in order to bring the coagulation to completion. This method, however, has a defect that it exerts adverse effects on the thermal stability of the powder as an article, and therefore that in order to solve the adverse effects, the coagulated slurry needs to be neutralized with an alkaline substance before solidification, which makes the coagulation process complicated.
The method of adding a coagulating agent in two or more steps has a defect that, when this method is used to coagulate the latex containing only a surface active agent unstable to acids, it locally brings the coagulation to completion even if the amount of the coagulated polymer and pH are controlled, because of which a good powdery and granular polymer is not obtained. Further, when the temperature is raised too high in a second coagulation vessel and vessels subsequent thereto, the coagulated particles are undesirably likely to aggregate.
Also, the method of modifying the surface of the coagulated particle by adding a hard resin or inorganic compound in order to raise the bulk specific gravity of the resulting powdery and granular polymer, intends to make dense the packing structure of the particle group, thereby raising the bulk specific gravity, and not to raise the particle density of the coagulated particle itself. This method, therefore, has a defect that a sufficiently high bulk specific gravity is not obtained.